using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Threading.Tasks;

using System.Threading;

using System.Net;

using System.Security.Cryptography;

​

public class Program

{

    public static async Task Main()

    {

        Dictionary<String, long> dummy = new Dictionary<String, long>();

        dummy.Add("foo", 0L);

        long foo = dummy["foo"];

        foo += 5;

        Console.WriteLine(dummy["foo"]); // 0

        dummy["foo"] += 5L;

        Console.WriteLine(dummy["foo"]); // 5

    }

}